This invention relates to a combination mill/drill bit having primary cutting elements and secondary structures, and more particularly to a combination mill/drill bit comprising primary cutting elements and secondary structures to enable continuous substantially smooth milling of down hole casing and drilling of an earth formation.
Heretofore, the milling of down hole casing and the drilling of an earth formation laterally from the casing were performed as two separate operations requiring tripping the mill tool before sidetrack drilling of laterals in the earth formation. Milling tools for casing have typically been made of crushed carbide to provide sufficient cutting ability for the varying hardness of steel casing. Typically, a mill tool would be retrieved after milling a window in the down hole casing and then discarded.
The manufacturing process used in making prior art milling tools provided no organization of the cutting structure; consequently, milling a window in a down hole casing became a lengthy and time consuming process. This is due to varying down hole conditions, operating parameters and applications. In many cases the quality of the manufacturing process for applying the cutting structure to a milling tool depended upon the skill of the person applying the cutting structure. Experience has shown that present milling tools if used after cutting a window in a down hole casing would not be effective in drilling a lateral bore in most earth formations. Thus the milling tool must be tripped out of the well, a time consuming and costly operation. After tripping out the milling tool a second tool, that is a drill bit, suited for drilling formations of the earth would be tripped into the well to proceed with the drilling portion of the operation.
Typically, the cutting element of an earth boring drill bit, although very wear resistant, could not survive the impacts of the interrupted cuts that occur during milling a window in steel casing. Thus the earth boring drill bit cannot be used for both the milling operation and the drilling operation. When drill bits designed for boring in various earth formations were used for milling a window in a casing, the result was usually catastrophic in that most of the cutting elements were broken primarily resulting from the impact loads as the cutting elements enter and leave the steel casing during the window milling operation.
As a result of varying down hole conditions, performance of present milling/drilling tools used to mill a casing window and then laterally drill through an earth formation produced varying results. To achieve some degree of uniformity required extensive control of manufacturing processes to ensure quality tools considered to be essential for consistent performance. The results have not always been encouraging and most operators prefer to trip the milling tool and run in the drill bit even though this was a time consuming and costly operation. Thus, there is a need for a mill/drill bit that will remain in the bore hole after window milling of the steel casing and continue with the drilling of laterals in the earth formation.
In accordance with the present invention there is provided a drag-type mill/drill bit for down hole milling of a window in casing and lateral drilling of an earth formation that comprises a bit body adapted to be rotated in a defined direction. The bit body has an operating end face with a plurality of primary cutting elements each mounted in a pocket in the operating end face of the bit body. In addition, a plurality of secondary ridge structures are mounted to the operating end face interspersed with the plurality of primary cutting elements in a pattern such that as the bit body rotates the secondary ridge structures contact the down hole casing or the earth formation thereby protecting the primary cutting elements and allowing continuous substantially smooth casing milling or earth formation drilling.
Also in accordance with the present invention there is provided a drag-type mill/drill bit for a down hole milling of a window in casing and lateral drilling of an earth formation comprising a bit body adapted to be rotated in a defined direction, the bit body having an operating end face. A plurality of radially extending blades are formed as a part of the operating end face of the drill bit and a plurality of primary cutting elements are mounted in pockets in the plurality of blades. A plurality of secondary ridge structures are also mounted to each of the plurality of blades and interspersed with the plurality of primary cutting elements in a pattern such that as a bit body rotates the secondary ridge structures contact the down hole casing or the earth formation thereby protecting the primary cutting elements and allowing continuous substantially smooth casing milling or earth formation drilling.
The primary cutting elements and the secondary ridge structures are arranged about the bit face to increase element contact during operation, particularly when interrupted cuts are taken while milling a window in steel casing. The secondary ridge structures may be configured in a pattern of full or partial concentric rings or other beneficial shapes. The secondary ridge structures are positioned such that as the bit body rotates, one secondary ridge structure protrusion slides off the steel or rock and another secondary ridge structure protrusion comes in contact, allowing continuous smooth drilling.
In another embodiment of the invention, the secondary ridge structures are mounted adjacent to a primary cutting element to control the cutting depth of the primary cutting element. The leading edge of the secondary ridge structure would be positioned to expose the front face of the primary cutting element. Each secondary ridge structure has a leading edge configuration ramping up to the desired height.
Technical advantages of the mill/drill bit of the present invention include utilizing the same primary cutting elements for both window milling of steel casing and drilling in earth formations of varying hardness. This results in a substantial cost saving by eliminating trip-out and trip-in when boring multiple laterals from the same casing. By use of the secondary ridge structures that allow continuous substantially smooth casing milling or earth formation drilling, there is achieved the technical advantage of reliable window milling in steel casing and improved efficiency in lateral bore hole drilling.